The goal of this proposal is to understand the sex determination process in a model genetic system Zea mays. Sex determination in maize involves the formation of unisexual florets derived from a bisexual floral meristem. Unisexual florets arise through the targeted elimination and selective maturation of floral organ primordia by processes involving three distinct mechanisms: (i) cell death of gynoecia, (ii) cell arrest of stamen primordia, and (iii) cell protection in functional pistils. Genes involved in the regulation and elicitation of cell death, cell arrest, and cell protection have been identified in this sex determination pathway. A formal genetic characterization of these genes and gene products will be conducted to provide basic information about these processes at the cellular and biochemical levels. The specific goals focus on the genetic and molecular characterization of three key steps in this pathway-cell death signaling by tassel seed 2, the regulation of the cell death pathway by tassel seed 1, and the protection from cell death by silkless 1. In the course of these investigations, several specific questions and hypotheses will be tested, including: (i) determining whether or not Ts2 expression is sufficient to sinal a cell death response in cultured cells; (ii) testing the hypothesis that Ts2 is a direct target of Ts1 transcriptional regulation; (iii) investigating whether or not the SK1 product can directly or indirectly interact with TS2 protein. Lastly, investigations are outlined to test the hypothesis that unisexuality may have originated by recruiting genes from a general cell death pathway in plants using the model plant system Arbidopsis thaliana. Cell death, cell protection and cell arrest represent some of the most intensively studied areas of molecular medicine, implicated in development of limbs, maturation of the immune and nervous systems, cavitation, and aging. Misregulation of cell death and arrest may be responsible for such human afflictions as AIDS, Alzheimer's, heart disease and stroke, cancers, and responses to pathogen attack. Extending these studies to plants will provide an independent and novel approach to understanding why and how cells are programmed to die, while providing a non-animal system for the study of these medically relevant topics.